Atumnal Equinox Follow Up

Today is the Equilux, and three days ago we gave the following facts about the Equinox (they're not the same), and then posed the puzzle below:

  • We tend to say that a particular day is the equinox, when in fact the equinox is a moment in time when the sun is directly overhead at the equator - technically the celestial equator.
  • This also means that the equinox will be on different days in different parts of the world, depending on your time zone. Those countries in the east (just to the west of the International Date Line) often have the equinoxes during the day after those countries further west. The equinox is at the same time, it just happens to be after midnight in those countries.
  • Most other planets in the solar system have equinoxes. On Saturn the rings align with the sun's rays and they cast virtually no shadow.
  • Equinox comes from the Latin for equal night, but this doesn't mean that everywhere gets a day and night of the same length. Because of diffraction in the atmosphere, most places will have more than twelve hours of daylight on the equinox.

This last point leads to an interesting question: In any given year, does everywhere on the surface earth get the same amount of daylight (and thus the same amount of darkness) as every other place? We don't mean sunshine, we mean daylight hours between sunrise and sunset.

Well, the answer is No.

Firstly, summer is shorter in the southern hemisphere than the northern, as the perihelion (when the earth is closest to the Sun) is during the southern summer. The earth moves faster the nearer it is to the sun, and so the summer is a few days shorter. So more daylight in the north.

Secondly, diffraction causes daylight to extend "beyond the horizon" giving everywhere a little more daylight than darkness. This is most pronounced at the poles, so the North Pole gives the most daylight of anywhere on earth.


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